To shan someone is to sexually harass a person in a social setting such as in a club, a bar, a house party, ... in an unashamed and perverted fashion. Often done by virgins and closet gays as a way to appear as "playas" in front of their peers.

Dude I went clubbing with this douchebag the other day, and he just totally attempted to shan some girl. She ran away crying and he was like "bro she's totally lesbo, she's lucky I even wanted to shan her".

A word English chavs think is their own unique word but it's just another from Scots which has been in regular use in Scotland for hundreds of years. Earliest example from the work of Allan Ramsay, a Scots makar from the 17th century.

Has a variety of meanings including worn-out, unfair, frightened, and more. Of obscure origin. Possibly from old Scottish Gaelic 'sean', meaning old, worn, ancient.

yeah man, that's shan innit?!
using Scots words again, minger?
wot you on abaht? this is ours! we invented it man!